Motion Of Screw Dislocations Research Articles

First-principles calculations of interaction between solutes and dislocations in tungsten

Changes in mechanical properties due to transmutation products (Re and Os) in W alloys is a central issue for plasma-facing materials in fusion reactors. We conducted density functional theory calculations to investigate the effect of Re, Os, and other 5d solutes on core structure and motion of screw dislocations associated with plastic deformation. Ir, Pt, Au, and Hg solutes show strong attractive interactions with screw dislocations, causing solution strengthening by the pinning mechanism. On the other hand, Hf, Ta, and Re cause softening by facilitating dislocation motion around solutes. This prediction corresponds well with the experimental observation of softening behavior in W-Re alloys.

Dislocation mobility and Peierls stress of c-type screw dislocations in GaN from molecular dynamics

Motion of a single c-type screw dislocation, with [0001] Burgers vector, in single crystal GaN is explored using classical molecular dynamics (MD) simulations with both Tersoff and Stillinger-Weber (SW) interatomic potentials. Three different dislocation core structures (full, small void, and open) along two distinct glide planes ((11¯00) and (1¯1¯20)) are studied. The system is sheared by applying opposing forces to atoms near the surface, with interior atoms maintained at a temperature of 1300 K. This results in glide for screw dislocations with a full core structure, with velocities dependent on the choice of interatomic potential. Full core dislocation glide also results in the nucleation of void defects for both potentials. Small void dislocations glide cleanly with no debris using the SW potential, but glide is not observed for the Tersoff potential. The difference between the potentials is most fully demonstrated from the response of open core dislocations in the (1¯1¯20) slip system. For the Tersoff potential, full core screw dislocations are nucleated from the surface of the open core dislocations, while dislocation glide loops are nucleated for the SW potential. The discrepancy in results using the different potentials is discussed, and attributed to the short cut off distance of the Tersoff potential. Finally, a dislocation dipole system is used to calculate the Peierls stress of the {11¯00}〈0001〉 full core screw dislocation, and compared to experimental results.

The role of twinning and nano-crystalline ω phase on the fatigue behavior of the metastable β Ti-15Mo alloy

This work evaluated in depth the fatigue and fracture behavior of the metastable β Ti-15Mo alloy considering the presence of deformation twins and athermal nano-crystalline ω phase in the microstructure. Regardless of the microstructural condition studied, the as-received and the solution treated and quenched materials, ω phase remained unchanged, guaranteeing the static mechanical properties at acceptable levels. The mechanism of fatigue fracture was related to the existence of twins and maximum shear stress planes near the direction of screw dislocation motion in the BCC structure. The significant amount of deformation twins in the initial microstructure did not alter the fatigue limit, because the solution treated and quenched material unexpectedly developed twins during the fatigue test.

Direct prediction of the solute softening-to-hardening transition in W–Re alloys using stochastic simulations of screw dislocation motion

Interactions among dislocations and solute atoms are the basis of several important processes in metal plasticity. In body-centered cubic (bcc) metals and alloys, low-temperature plastic flow is controlled by screw dislocation glide, which is known to take place by the nucleation and sideward relaxation of kink pairs across two consecutive Peierls valleys. In alloys, dislocations and solutes affect each other’s kinetics via long-range stress field coupling and short-range inelastic interactions. It is known that in certain substitutional bcc alloys a transition from solute softening to solute hardening is observed at a critical concentration. In this paper, we develop a kinetic Monte Carlo model of screw dislocation glide and solute diffusion in substitutional W–Re alloys. We find that dislocation kinetics is governed by two competing mechanisms. At low solute concentrations, nucleation is enhanced by the softening of the Peierls stress, which dominates over the elastic repulsion of Re atoms on kinks. This trend is reversed at higher concentrations, resulting in a minimum in the flow stress that is concentration and temperature dependent. This minimum marks the transition from solute softening to hardening, which is found to be in reasonable agreement with experiments.

The Filon construct for moving dislocations

Filon’s construct, originally developed for plane isotropic linear elastostatics, examines the difference between solutions to the same boundary value problem but for two different values of the elastic moduli, and relates the difference solution to one occurring in a corresponding stationary dislocation problem. This paper generalises the result to three-dimensional linear elastodynamics with particular reference to moving dislocations. Also studied is the inverse procedure which derives the pair of elastodynamical problems from a given distribution of dislocations. Body-forces and an auxiliary plastic distortion tensor are novel and essential features of the argument. Specialisation to the static and quasi-static theories is straightforward. The inverse Filon construct is illustrated by the stationary edge dislocation and uniformly moving screw dislocation in a homogeneous isotropic linear elastic whole space.

Microstructure evolution and superelasticity of layer-like NiTiNb porous metal prepared by eutectic reaction

This work combines several techniques into a novel method to manufacture NiTiNb layer-like porous structures with 3D interconnected channels, which are of vital importance in the biomedical field. NiTi wires and Nb foils are used herein to prepare NiTiNb layer-like porous materials via in situ eutectic reaction. Microstructure, phase transformation and superelasticity of the resultant NiTiNb layer-like porous material are analyzed and discussed, and the interface between NiTi wires and the solidified NiTiNb eutectic region are also studied. Dislocations and stacking faults occurring at interfaces accelerate the eutectic phase transformation. Meanwhile, rod-like eutectic phase pins the motion of screw dislocations, which is beneficial for stress-induced R martensite nucleation. It is believed that much more superelastic recovery and elastic recovery for such a layer-like porous structure are attributed to formation of more martensites during deformation. Furthermore, both reorientation and subsequent de-twinning of <011> type II twins result in the observed stress-plateau, thereby promoting the superelastic recovery.

Minimising movements for the motion of discrete screw dislocations along glide directions

In Alicandro et al. (J Mech Phys Solids 92:87–104, 2016) a simple discrete scheme for the motion of screw dislocations toward low energy configurations has been proposed. There, a formal limit of such a scheme, as the lattice spacing and the time step tend to zero, has been described. The limiting dynamics agrees with the maximal dissipation criterion introduced in Cermelli and Gurtin (Arch Ration Mech Anal 148, 1999) and predicts motion along the glide directions of the crystal. In this paper, we provide rigorous proofs of the results in [3], and in particular of the passage from the discrete to the continuous dynamics. The proofs are based on $$\Gamma $$ -convergence techniques.

Elastodynamic image forces on screw dislocations in the presence of phase boundaries.

The elastodynamic image forces acting on straight screw dislocations in the presence of planar phase boundaries are derived. Two separate dislocations are studied: (i) the injected, non-moving screw dislocation and (ii) the injected (or pre-existing), generally non-uniformly moving screw dislocation. The image forces are derived for both the case of a rigid surface and of a planar interface between two homogeneous, isotropic phases. The case of a rigid interface is shown to be solvable employing Head's image dislocation construction. The case of the elastodynamic image force due to an interface is solved by deriving the reflected wave's contribution to the global solution across the interface. This entails obtaining the fundamental solution (Green's function) for a point unit force via Cagniard's method, and then applying the convolution theorem for a screw dislocation modelled as a force distribution. Complete, explicit formulae are provided when available. It is shown that the elastodynamic image forces are generally affected by retardation effects, and that those acting on the moving dislocations display a dynamic magnification that exceed the attraction (or repulsion) predicted in classical elastostatic calculations.

On the plasticity mechanisms of lath martensitic steel

The plasticity mechanisms of press hardening steel with a fully lath martensite microstructure were examined experimentally by strain rate sensitivity measurements, repeated relaxation tests and internal friction measurements. The analysis of relaxation tests suggests that the micro-plasticity could be due to the motion of mobile non-screw dislocations, based on mobile dislocation exhaustion observed in the micro-plastic range. In the macro-plastic range, the plasticity is thought to be due to the generation of mobile screw dislocations. The solute carbon-dislocation interaction results in a negative strain rate sensitivity and a Snoek-Köster-Kê peak in the internal friction spectrum of the lath martensitic press hardening steel. The magnitude of the effective activation volume and its stress dependence indicate that plastic deformation is most likely controlled by screw dislocation motion by formation and lateral movement of kink pairs dragging solute carbon atom atmospheres. Both isotropic and kinematical hardening seem to play a role in the strain hardening behavior of lath martensitic steel.

Chemical misfit origin of solute strengthening in iron alloys

In this first-principles study, we investigate the effect of many kinds of substitutional solute species on screw dislocation motion in bcc-Fe, dominating the strength of dilute Fe alloys. Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A first-principles interaction energy with the “Easy-core” structure excellently correlates with the change in the γ-surface caused by solute atoms (i.e., chemical misfit). Based on the interaction energy, we predicted the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles.

Dislocation cross-slip in fcc solid solution alloys

Cross-slip is a fundamental process of screw dislocation motion and plays an important role in the evolution of work hardening and dislocation structuring in metals. Cross-slip has been widely studied in pure FCC metals but rarely in FCC solid solutions. Here, the cross-slip transition path in solid solutions is calculated using atomistic methods for three representative systems of Ni-Al, Cu-Ni and Al-Mg over a range of solute concentrations. Studies using both true random alloys and their corresponding average-alloy counterparts allow for the independent assessment of the roles of (i) fluctuations in the spatial solute distribution in the true random alloy randomness and (ii) average alloy properties such as stacking fault energy. The results show that the solute fluctuations dominate the activation energy barrier, i.e. there are large sample-to-sample variations around the average activation barrier. The variations in activation barrier correlate linearly with the energy difference between the initial and final states. The distribution of this energy difference can be computed analytically in terms of the solute/dislocation interaction energies. Thus, the distribution of cross-slip activation energies can be accurately determined from a parameter-free analytic model. The implications of the statistical distribution of activation energies on the rate of cross-slip in real alloys are then identified.

Upscaling a Model for the Thermally-Driven Motion of Screw Dislocations

We formulate and study a stochastic model for the thermally-driven motion of interacting straight screw dislocations in a cylindrical domain with a convex polygonal cross-section. Motion is modelled as a Markov jump process, where waiting times for transitions from state to state are assumed to be exponentially distributed with rates expressed in terms of the potential energy barrier between the states. Assuming the energy of the system is described by a discrete lattice model, a precise asymptotic description of the energy barriers between states is obtained. Through scaling of the various physical constants, two dimensionless parameters are identified which govern the behaviour of the resulting stochastic evolution. In an asymptotic regime where these parameters remain fixed, the process is found to satisfy a Large Deviations Principle. A sufficiently explicit description of the corresponding rate functional is obtained such that the most probable path of the dislocation configuration may be described as the solution of Discrete Dislocation Dynamics with an explicit anisotropic mobility which depends on the underlying lattice structure.

Screw-Dislocation-Induced Strengthening-Toughening Mechanisms in Complex Layered Materials: The Case Study of Tobermorite.

Nanoscale defects such as dislocations have a profound impact on the physics of crystalline materials. Understanding and characterizing the motion of screw dislocation and its corresponding effects on the mechanical properties of complex low-symmetry materials has long been a challenge. Herein, we focus on triclinic tobermorite, as a model system and a crystalline analogue of layered hydrated cement, and report for the first time how the motion of screw dislocation can influence the strengthening-toughening relationship, imparting brittle-to-ductile transitions. By applying shear loading in tobermorite systems with single and dipole screw dislocations, we observe dislocation jogs around the dislocation core, which increases the yield shear stress and the work-of-fracture when the dislocation lines are along the [100] and [010] directions. Our results demonstrate that the dislocation core acts as a bottleneck for the initial straight gliding to induce intralaminar gliding, which consequently leads to a significant improvement in the mechanical properties. Together, the fundamental knowledge gained in this work on the role of the motion of the dislocation core on the mechanical properties provides an improved understanding of deformation mechanisms in cementitious materials and other complex layered systems, providing new hypotheses and design guidelines for the development of strong, ductile, and tough materials.

Dynamics of screw dislocations: A generalised minimising-movements scheme approach

The gradient flow structure of the model introduced in Cermelli &amp; Gurtin (1999, The motion of screw dislocations in crystalline materials undergoing antiplane shear: glide, cross-slip, fine cross-slip. Arch. Rational Mech. Anal.148(1), 3–52) for the dynamics of screw dislocations is investigated by means of a generalised minimising-movements scheme approach. The assumption of a finite number of available glide directions, together with the “maximal dissipation criterion” that governs the equations of motion, results into solving a differential inclusion rather than an ODE. This paper addresses how the model in Cermelli &amp; Gurtin is connected to a time-discrete evolution scheme which explicitly confines dislocations to move at each time step along a single glide direction. It is proved that the time-continuous model in Cermelli &amp; Gurtin is the limit of these time-discrete minimising-movement schemes when the time step converges to 0. The study presented here is a first step towards a generalisation of standard gradient flow theory that allows for dissipations which cannot be described by a metric.

Low temperature in-situ micro-compression testing of iron pillars

An in-situ nanomechanical cooling system has been developed to study the temperature dependency of local mechanical properties and slip behavior of bcc α-Fe. Uniaxial compression tests with Focused Ion Beam (FIB) fabricated pillars with a diameter of 1µm, were performed in the single slip orientations 2¯35 and 1¯49 at room temperature and 198K. The testing was conducted inside a Scanning Electron Microscope (SEM) equipped with a nanoindenter. Slip trace analyses revealed occurrence of slip in the {112}<111> family of slip systems for 2¯35 pillars at both room temperature and 198K while the predominantly slip systems governing the deformation on 1¯49 pillars were {110}<111> for both test temperatures. The stress-strain response showed an increased strength with decreasing temperature for the 2¯35 pillars, in contrast to 1¯49 pillars, where only a weak temperature dependence is observed. Furthermore, for 2¯35 pillars, the appearance of slip is less prominent at 198K, indicating that the temperature strongly influences the relative motion of screw and edge dislocations. Molecular Dynamics (MD) simulations performed at 15K and 300K, was used to study dislocation mechanisms for the two orientations. 1¯49 pillars exhibit a change in deformation mechanisms at low temperature and the evolution of dislocation density during deformation, display distinct differences for the two loading orientations.

Oxide particle–dislocation interaction in 9Cr-ODS steel

Oxide Dispersion Strengthened (ODS) ferritic/martensitic steels have an excellent high temperature strength primarily due to a dislocation pinning effect of nanometric oxide particles. In the present work, the interaction between oxide particles and dislocations in 9CrODS ferritic steel was investigated by both static TEM observation and in-situ TEM observation under dynamic straining conditions. The primary concerns of those observations were the obstacle strength of oxide particles and the type of interactions: attractive or repulsive. In the static observation, the majority (∼90%) of all interaction geometries was characterized as repulsive type. In the in-situ straining experiments, the obstacle strength α of oxide particles was estimated to be no greater than 0.80. The experimentally-determined obstacle strength is smaller than that of Orowan type impenetrable obstacle, whereas those oxide particles are, in theory, ideally strong obstacles. The gap between predicted and measured obstacle strength is attributable to cross-slip motion of screw dislocations on the oxide particles.

Twin Boundaries merely as Intrinsically Kinematic Barriers for Screw Dislocation Motion in FCC Metals.

Metals with nanoscale twins have shown ultrahigh strength and excellent ductility, attributed to the role of twin boundaries (TBs) as strong barriers for the motion of lattice dislocations. Though observed in both experiments and simulations, the barrier effect of TBs is rarely studied quantitatively. Here, with atomistic simulations and continuum based anisotropic bicrystal models, we find that the long-range interaction force between coherent TBs and screw dislocations is negligible. Further simulations of the pileup behavior of screw dislocations in front of TBs suggest that screw dislocations can be blocked kinematically by TBs due to the change of slip plane, leading to the pileup of subsequent dislocations with the elastic repulsion actually from the pinned dislocation in front of the TB. Our results well explain the experimental observations that the variation of yield strength with twin thickness for ultrafine-grained copper follows the Hall-Petch relationship.

Mechanical and Thermophysical Properties of Cerium Monopnictides

The ultrasonic attenuation due to phonon–phonon interaction, thermoelastic relaxation and dislocation damping mechanisms has been investigated in cerium monopnictides CeX (X: N, P, As, Sb and Bi) for longitudinal and shear waves along \({\langle }100{\rangle }\), \({\langle }110{\rangle }\) and \({\langle }111{\rangle }\) directions. The second- and third-order elastic constants of CeX have also been computed in the temperature range 0 K to 500 K using Coulomb and Born–Mayer potential upto second nearest neighbours. The computed values of these elastic constants have been applied to find out Young’s moduli, bulk moduli, Breazeale’s non-linearity parameters, Zener anisotropy, ultrasonic velocity, ultrasonic Gruneisen parameter, thermal relaxation time, acoustic coupling constants and ultrasonic attenuation. The fracture/toughness ratio is less than 1.75, which shows that the chosen materials are brittle in nature as found for other monopnictides. The drag coefficient acting on the motion of screw and edge dislocations due to shear and compressional phonon viscosities of the lattice have also been evaluated for both the longitudinal and shear waves. The thermoelastic loss and dislocation damping loss are negligible in comparison to loss due to Akhieser damping (phonon–phonon interaction). The obtained results for CeX are in qualitative agreement with other semi-metallic monopnictides.

第一原理計算に基づく鉄中のらせん転位運動に対する炭素原子の影響解析

第一原理計算に基づく鉄中のらせん転位運動に対する炭素原子の影響解析

Calculation of stress intensity factor in a non-homogeneous orthotropic half-plane weakened by moving cracks

The dynamic behavior of multiple moving cracks with constant length propagating in the non-homogeneous orthotropic half-plane under anti-plane loading is considered. The Galilean transformation is employed to express the wave equations in terms of coordinates that are attached to the moving crack. Finally, the solution of a moving screw dislocation is obtained in a non-homogeneous orthotropic half-plane. The solution is employed to derive integral equations for a half-plane weakened by several moving cracks. Numerical examples are provided to show the effects of material properties, crack size and the speed of cracks propagating on the stress intensity factors of crack tips.